Distributed fibre optic temperature sensing systems are currently being employed in the field to monitor fluid-carrying pipelines and detect leaks therein. In a typical deployment, a distributed temperature sensing (DTS) system includes fibre optic sensing cables laid out in a sensing line that runs on or alongside sections of the pipeline to be monitored and a DTS unit to which the sensing cables are optically coupled. The DTS unit emits pulses of light through the sensing cables and receives backscattered light signals. These light signals are processed using ODTR techniques (Optical Time Domain Reflectometry) to derive therefrom temperature values associated with locations along the sensing line.
In a gas-carrying or liquefied gas-carrying pipeline, when a leak occurs at a location along the sensing line the DTS system will generally detect a localized temperature drop or “cold spot”. The gaseous substance flowing through the pipeline is cooled down by the pressure release through the leaking pipe section and cools the pipe section and the surrounding area. In some cases, it is also possible to observe a hot-spot when a leak occurs in a gas-carrying pipeline, for example, in a pipeline carrying water vapor.
In a liquid-carrying pipeline, when a leak occurs at a location along the sensing line the DTS system will generally detect a localized temperature rise or “hot spot”. The liquid substance flowing through the pipeline is typically warmer than the structures adjacent the sensing line such that when the liquid escapes from the pipeline it tends to warm such structures. In some cases, it is also possible to observe a cold-spot in the case of leaks from a liquid-carrying pipeline, for example, if the transported liquid is colder than the environment.
Such DTS systems have the advantage of being scalable and relatively easy to deploy over long pipeline sections. Moreover, such systems have been shown to detect leakage events with good accuracy and reliability. However, as with all safety systems, it is important to be able to assess the reliability of the DTS system and to test whether the DTS system is functioning properly. In the past, such assessment and testing have been performed periodically on annual or quarterly basis by pipeline personnel. Such periodic testing typically involves a worker manually exposing the sensing line to a cooling or heating source to produce a localized cold or hot spot. This is not unlike the periodic testing performed in fire detection systems and LNG pipelines.
It would be advantageous to be able to assign a Safety Integrity Level (SIL) or equivalent confidence level to such DTS systems. However, in order for the DTS system to be SIL-certified, certain requirements have to be met, inter alia, targets for maximum probability of a dangerous failure. These requirements can be complied with by establishing a rigorous development and documentation process, or by establishing that the system has sufficient operating history to demonstrate that it has been proven in use. In some cases, due to the complexity of software used to operate DTS systems, it may not be possible to demonstrate compliance with SIL certification requirements by way of a rigorous development and documentation process. Accordingly, in such cases, the only way to show compliance with SIL certification or equivalent confidence level requirements is through extensive proof of use. With current testing of the DTS systems being performed manually only a few times a year, it is difficult to generate the data required to evidence the DTS system's reliability through proven use.
Based on the foregoing, it would be advantageous if a DTS system could be provided with an independent testing system that could easily be incorporated into a sensing line and that would be operable to test the reliability and functionality of the DTS system on a relatively high-frequency basis in a continuous and autonomous manner.